The present invention relates to an arrangement for guiding and shifting a carriage of an office machine, for example a digital document reader or a parallel dot printer.
In office machines of the aforesaid type, the document to be read or the sheet to be printed remains still on a plate and is explored in sequence by a carriage which must shift parallel to itself step by step with great precision. Since the shift step corresponds to an elementary scanning line, this step must be all the smaller the greater the reading or printing resolution.
In known machines, such carriages are guided on prismatic guides, while advance is commanded through separate means, as a result of which, because of occasional friction and resistance, the carriage tends to skew. The carriage is moreover of complicated and costly construction.
An arrangement is already known (U.S. Pat. No. 4,268,870) in which the fixed guide of the carriage is constituted by a pair of worms parallel to one another and rotated simultaneously, which engage two nuts borne by the carriage.
Machined worms (lead screws) are expensive and it is difficult to avoid backlash. It is already know (U.S. Pat. No. 2,724,979) to construct a screw from a deformed sheet metal tube supported on a core tube. Such a screw is not suited to high precision uses. It is also known to provide a screw with self-lubricating properties by coating the threads of the screw with a plastics layer (GB 2 104 998 and DE 872 701). The latter reference also discloses an alternative in which the threads of the nut are coated with a plastics layer. In these known structures it remains necessary to machine the base thread to which the coating is applied.